1. Field of the Invention
This invention relates to a rubber composition and a hose. More particularly, it relates to a zinc-free peroxide-cure system rubber composition comprising EPDM as a base material, and a hose formed therefrom and used in, for example, a cooling system for an automobile engine.
2. Description of the Related Art
A hose having an inner wall layer formed from a vulcanized rubber composition consisting mainly of EPDM is often used in a cooling system for an automobile engine, for example, with a radiator, or heater. The vulcanization of any such composition is typically carried out by using a peroxide cure system or sulfur cure system. For sulfur vulcanization, it is essential to use zinc oxide (zinc white) as an accelerator activator. Zinc oxide is understood to play an important role in peroxide vulcanization, too, since it acts as a radical scavenger to keep the composition resistant to heat during hot air aging.
Zinc oxide stays in the form of a zinc salt in a rubber composition after this composition has been vulcanized. It is, however, known that the zinc salt gradually migrates to the surface of the vulcanized product, such as a hose, dissolves into any coolant in the hose and reacts with phosphoric acid in the coolant to form an insoluble compound. The insoluble compound may cause various problems. It may eventually block the hose if it is deposited on its inner wall surface, or if it is deposited on a seal between the hose and any pipe connected therewith, it may cause the coolant to leak out therethrough.
Those problems are avoidable if there is a rubber composition not causing any such dissolution of zinc. They can, however, be overcome more easily and effectively if there is available a zinc-free composition not containing any zinc oxide. No sulfur-vulcanizable composition can, however, be zinc-free, since its vulcanization essentially requires zinc oxide as an activator. The zinc-free composition, therefore, relates to a peroxide cure system EPDM composition used for engine cooling hoses.
Zinc oxide plays an important role as a radical scavenger in a peroxide cure system EPDM composition as stated before, and it is, therefore, a first problem that a composition not containing zinc oxide may be less resistant to hot air aging. Although the addition of an antioxidant to the composition may usually be effective for keeping it resistant to heat aging, it is a second problem that in the peroxide cure system the antioxidant may consume the peroxide cure agent and thereby hinder its vulcanization.
In view of the first and second problems stated above, it is an object of this invention to provide a peroxide cure system rubber composition comprising EPDM as a base material, which is zinc-free and yet highly resistant to heat aging.
In connection with the first problem, the inventors of this invention have paid attention to the iodine value of EPDM in a rubber composition. It is generally true that EPDM having a high iodine value gives a composition of low heat resistance. It is, therefore, usual to use EPDM having an iodine value not exceeding 8 in a peroxide cure system composition which does not require its rubber component to have a high degree of unsaturation.
The inventors have, however, found experimentally that EPDM having a high iodine value gives a composition which is greatly resistant to any lowering of its physical properties after hot air aging, even if it may not contain zinc oxide, though it may be somewhat inferior in its physical properties in ordinary state such as elongation and hardness due to a high degree of unsaturation. As a result, the inventors have found that a hose formed from any such composition has an improved resistance to cracking by heat aging. The inventors have also found that keeping the proportion of ethylene in a composition containing EPDM within a specific range has a favorable influence on the seal at joints of hoses, maintaining a low compression set at low temperatures.
In connection with the second problem, the inventors have found that the use of EPDM having a high iodine value makes it possible to reduce the action of any antioxidant against vulcanization considerably. Quinoline is the most preferable antioxidant to be used with a peroxide cure system EPDM composition.
According to a first aspect of this invention, there is provided a rubber composition comprising an ethylene-propylene-diene terpolymer (EPDM) as a base material and a peroxide type vulcanizing agent, in which the EPDM has an iodine value of 10 to 24 and an ethylene proportion of 48 to 60% by weight.
The rubber composition according to the first aspect of this invention gives a vulcanized product which is greatly resistant to any lowering of its physical properties by heat aging, since the EPDM which it contains has an iodine value of as high as 10 to 24, though it does not contain zinc. Firstly, the EPDM having a high iodine value makes it easy to obtain a zinc-free composition. The vulcanized product of the composition can be used to form any of various objects that may be exposed to an engine coolant without causing any problem with insoluble compounds of zinc. More specifically, it can be used to form, for example, the inner wall layer of a hose in a cooling system for an engine and thereby avoid any blocking of the hose or any leakage of the coolant through any seal associated therewith. Secondly, the EPDM having a high iodine value makes it possible to obtain a vulcanized product which is greatly resistant to any lowering of its physical properties, such as tensile strength (TB) and elongation at break (EB), by heat aging, though it may be somewhat inferior in these physical properties in ordinary state. The vulcanized product has a greatly improved flex life after heat aging. More specifically, the occurrence of cracking of a hose in a cooling system for an engine is greatly suppressed. The improvement is apparently due to a large number of crosslinks in the vulcanized product of a composition containing EPDM having a high iodine value.
As EPDM has an ethylene proportion of 48 to 60% by weight, the vulcanized product of the composition according to this aspect has a satisfactorily low compression set at low temperatures, so that a hose in a cooling system for an engine or the like can maintain a seal of improved tightness at its joint with a pipe.
According to a second aspect of this invention, the rubber composition further contains an antioxidant. The antioxidant improves the heat resistance of the vulcanized product of the. composition. It can be added to the peroxide cure system composition without causing any appreciable harm, since EPDM having a high iodine value is effective for canceling out the action of the antioxidant against vulcanization considerably, as mentioned before.
According to a third aspect of this invention, the antioxidant is quinoline, or a combination of at least two antioxidants including quinoline.
When used alone as the antioxidant, quinoline is particularly effective for improving the heat resistance of the vulcanized product, and also hinders vulcanization still less, for some reason not known as yet.
According to a fourth aspect of this invention, the composition contains the antioxidant of the second or third aspect in an amount not exceeding 3 parts by weight per 100 parts by weight of EPDM. Any excess of the antioxidant is likely to result in its action against vulcanization which may not be negligible, even though the composition may contain EPDM having a high iodine value.
According to a fifth aspect of this invention, there is provided a hose formed from a rubber composition according to any of the first to fourth aspects of this invention, and used in a cooling system for an engine. The hose is of high heat resistance (particularly, high hot air aging resistance), and can maintain a tight seal at a joint, while having other advantages owing to the absence of zinc.
The above and other features and advantages of this invention will become more apparent from the following description and the accompanying drawing.